1. Field of the Invention
This invention relates to a method and apparatus for the treatment of flue gases containing dust and/or pollutants.
2. Description of the Related Art
In known processes for electrostatic purification of dust- and pollutant-containing exhaust gases, the latter are subjected to an electrostatic purification under dry conditions in a first processing stage and in a succeeding second processing stage are subjected to an electrostatic purification under wet conditions. One or more electric fields operating under dry conditions, and one or more electric fields operating under wet conditions are arranged in succession. The water is sprayed through nozzles into the wet field or fields and is drained as a slurry, which is concentrated in thickeners and is then injected by means of steam or compressed air into a drying tower, in which the evaporated liquid humidifies the hot drying gas so that a back corona discharge in the fields operating under dry conditions is prevented. Acid components, such as SOx, HF and HCl, are absorbed from the liquid which has been sprayed into the wet stage and, together with the dust which is still collected in the wet stage, enter a sump disposed in the wet stage. A disadvantage of that process resides in that the sludge formed in the sump of the wet stage contains a relatively large amount of pollutants in addition to the dust, and for this reason, the sludge can be processed only with difficulty. A further disadvantage of that process resides in that the evaporated liquid which has been injected into the drying tower will moisten the dust- and -pollutant-containing exhaust gas so that its dew point temperature will be increased. Because the gas temperature is decreased at the same time, the temperature in the electrostatic precipitator will decrease below the dew point temperature so that a corrosion caused by the acid components of the exhaust gas cannot be avoided.
U.S. Pat. No. 1,766,422 also describes the electrostatic purification of dust- and pollutant-containing exhaust gases in a process in which the exhaust gas laden with dust and pollutants is first subjected to electrostatic purification under dry conditions and subsequently to electrostatic purification under wet conditions. In that process, the collecting electrodes of the wet electrostatic purification stage are wetted with a treating liquid. The electrostatic precipitator is operated at such a high gas velocity, that the particles of the fine fraction will be collected in the dry electrostatic purification stage, and those of the coarse fraction will be collected in the wet electrostatic purification stage. In that process, the sludge formed in the sump of the wet electrostatic purification stage will contain a relatively large amount of pollutants in addition to the dust. An additional disadvantage of that process resides in that the exhaust gas is passed through the electrostatic precipitator at a relatively high gas velocity to ensure that the coarse particles of the dust contained in the exhaust gas can be collected in the wet electrostatic purification stage. As a result, the residence time of the exhaust gas in the wet electrostatic purification stage is not sufficient to ensure that the pollutants contained in the exhaust gas will be removed to a sufficient degree.
U.S. Pat. No. 4,364,910, Willet et al., issued Dec. 21, 1982, discloses a similar system and process for flue gas processing, to remove both gaseous contaminants, such as sulfur dioxide, and particulate matter such as flyash. This process integrates spray scrubbing apparatus, and wet electrostatic precipitation apparatus, and provides for the advantageous extraction and utilization of heat present in the flue gas which is being processed. The integrated system and process utilize a spray scrubbing tower into which the flue gas in introduced and into which aqueous alkali slurry is introduced as spray for sulfur dioxide removal therein. The flue gas leaves the tower in a moisture-laden state, and enters a wet electrostatic precipitator which includes a heat exchanger, where flyash and entrained droplets in the flue gas are removed by electrostatic precipitation and that is removed from the flue gas. The cleaned flue gas exits from the precipitator and discharges into a stack. The wet electrostatic precipitator of the integrated system and process includes a portion constructed as a cross-flow heat exchanger, with flue gas saturated with water vapor moving vertically upward inside tubes arranged in a staggered pattern, and ambient air being pulled horizontally across the outside of those tubes to cool the tube walls and thereby remove heat from the flue gas and cause condensation of water vapor on the inside wall surfaces. The condensate washes the electrostatically collected flyash particles down from the inside tube walls.
U.S. Pat. No. 4,776,391, Warner, issued Oct. 11, 1988, discloses a flue gas cleaning system, whereby an exhaust gas containing sulfur trioxide is passed through a first heat exchanger which cools the gas to a temperature which is above the sulfur trioxide dew point, so that condensation of sulfur trioxide does not occur in the first heat exchanger, but which is below a material-limit operating temperature of a second heat exchanger, which further cools the gas below the sulfur trioxide dew point, whereby the first heat exchanger is protected against corrosion and the second heat exchanger is protected against thermal damage.
U.S. Pat. No. 3,284,990, discloses a method of improving the electrostatic precipitation of particles by adding phosphorous pentoxide to the particles prior to their electrostatic precipitation.
U.S. Pat. No. 3,665,676 discloses a system to condition the particles of boiler flue gas by the use of a salt solution, such as a solution of ammonium sulfate or ammonium bisulfate. The salt solution is injected into the flue gas prior to entering the electrostatic precipitator, and the system includes a metering means for controlling the amount of conditioner injected into the flue gas. U.S. Pat. No. 3,665,676 indicates that, if desired, conventional automatic controls can be provided to open the metering means when the flue gas reaches the desired operating temperature, or to close it should the temperature fall below operating temperature. In addition, automatic controls can also be made to open the metering means to provide the amount of conditioner needed in proportion to the volume of gas to be conditioned.
U.S. Pat. No. 4,885,139, Sparks, issued Dec. 5, 1989, discloses a method for improving the cost-effectiveness of sulfur oxide- and particulate matter-removal, placing a sulfur oxide- or other acidic gas-removal system and a multi-stage electrostatic precipitator within a single housing. The removal system works by spraying a neutralizing slurry or solution into incoming flue gas to form neutral salts which dry in a reaction zone provided between the removal system and the electrostatic filtration module. This system also provides for simple retrofitting of existing systems to include SO2— or other acidic gas-removal systems.
In U.S. Pat. No. 4,885,139 there is disclosed a wet electrostatic precipitator in which furnace gases containing dust are contacted with water or solution to saturate them with water vapor. The saturated gases pass to an electrical precipitator wherein the temperature of the gases is reduced sufficiently to cause condensation of some of the water vapor to form a stream of condensed liquid on collecting electrodes. The walls of the collecting electrodes are maintained at a temperature which is cooler than the gasses passing through, by circulation of external air assisted by blower means or by other suitable cooling means, to cause condensation of moisture thereon, sufficient to form a continuous stream of film to continually wash away collected materials.
U.S. Pat. No. 3,958,960, Bakke, issued May 25, 1976, discloses a precipitator which includes one or more main sections, each having a plurality of spaced, substantially parallel collection plates, with discharge electrodes being interposed in the spaces between the collection plates. The plates extend in the direction of flow of a gaseous medium through the precipitator from an inlet to an outlet. Sprays of water are continuously directed against the collection plate surfaces. The gaseous medium enters through a diverging inlet section in which are arranged several sets of baffles in longitudinal spaced relation. Each set of baffles includes at least two rows of channels extending at a slight angle to the vertical axis. The channels in each row are spaced apart a distance greater than their width, and the rows are offset so that each channel in a second row is opposite a space between channels in a first row. Sprayers direct water on the upstream surfaces of the baffles in each set, the sprayers being directed both in the direction of the flow of the gaseous medium and transverse thereto. Following the inlet section, there is a transverse electrostatic precipitator section in which sets of baffles are arranged transverse to the direction of flow of the gaseous medium upstream and downstream of the transverse discharge electrodes. Water sprays are continuously applied to the latter baffles. An extended discharge section is provided upstream and downstream of the main precipitator sections, including transverse baffles and electrodes for applying a field toward the baffles from the main sections along the path of the medium. The final section is a transverse electrostatic precipitator section defining a mist eliminator, the baffles of which may be sprayed intermittently as required. An outlet section following the mist eliminator section is provided with a set of baffles, the rows of which are in overlapping relation so that their is no “open flow”.
U.S. Pat. No. 5,137,546, Steinbacher, et al., issued Aug. 11, 1992, discloses a process and an apparatus for the electrostatic purification of dust- and pollutant-containing exhaust gases in multiple-field precipitators. The exhaust gases are first subjected in a first stage to an electrostatic purification under dry conditions in gas passages defined by plate-like collecting electrodes, and are subsequently passed in a second stage through one or more fields defined by liquid-wetted collecting electrodes, which define gas passages. The liquid which is supplied in the second stages at the top ends of the collecting electrodes is laterally discharged from the precipitator, and the substantially dry dust which is still collected in the second stage is fed to dust-receiving means.
U.S. Pat. No. 5,330,733, Baroni, et al., issued Jul. 19, 1994, discloses a method for removing pollutants from a combustion gas. A combustion gas is cooled a first time to about 110° C. in a first cooling chamber, up to 90% of the ash is removed in a first precipitator, and the gas is cooled again in a second cooling chamber to about 80° C.; in a reaction chamber, by energization by means of electrons, the sulfur dioxide and nitrogen oxides are converted into sulfuric acid and nitric acid and, with the addition of dry ammonia, the two acids are converted to salts of ammonia, and the latter are retained in a second precipitator and sent to a chimney; from the instant of the injection of the ammonia into the reaction chamber, the temperature of the gas is maintained at above 80° C. to reduce the thermochemical conversion of the sulfur dioxide to acidic sulfites and sulfates; in a subsequent injection chamber, hydrogen peroxide is injected into the gas to convert the residual sulfur dioxide into sulfuric acid which, reacting with the ammonia produces ammonia salts. The '733 patent claims a method for removing pollutants from a combustion gas taken from a boiler plant, with the combustion gas containing light ashes, sulfur dioxide and nitrogen oxides as pollutants. The method comprises: cooling the combustion gas in a heat exchanger to about 110° C.; removing at least most of the light ashes from the combustion gas in at least one electrostatic precipitator; energizing the combustion gas by means of electrons generated by impulsive electric fields to convert most of the sulfur dioxide and the nitrogen oxides into sulfuric acid and nitric acid, respectively, with the presence of residual sulfur dioxide; adding dry ammonia prepared in a gas environment and stored at a temperature of about 100° C. to the combustion gas to convert the sulfuric acids and nitric acids into the ammonium salts of these acids, producing a combustion gas containing ammonium sulfate, ammonium nitrate, sulfur dioxide, and ammonia; introducing hydrogen peroxide into the combustion gas to convert the residual sulfur dioxide into sulfuric acid, which reacts with the ammonia to produce ammonium sulfate; removing the ammonium slats from the combustion gas in an electrostatic precipitator; and passing the combustion gas to a chimney.
U.S. Pat. No. 3,745,751, Zey, et al., issued Jul. 17, 1973, discloses a sulfur dioxide collection system comprising ozone-producing electrodes upstream from a baghouse, electrostatic precipitator, or scrubber, etc., whereby ozone is produced and simultaneously reacted with sulfur dioxide in the gas stream, thereby forming sulfur trioxide which is converted to sulfuric acid mist prior to, or after entry into, the conventional collector. The collection system removes the acids mist (together with particulates, if any) from the flowing gas.